Every piece of deformed rebar sold in the United States carries a series of raised symbols rolled into the steel during manufacturing. These markings tell you four things in order: which mill made the bar, how big it is, what type of steel it’s made from, and its strength grade. Once you know the pattern, you can pick up any bar on a jobsite and identify it in seconds.
The Four Marks and Their Order
Starting from one end of the bar, the markings always appear in the same sequence along one of the longitudinal ribs. Here’s what each position tells you:
- First mark: producing mill. This is a letter or symbol that identifies the manufacturer. Each steelmaker has a unique logo or letter registered with the industry. You won’t memorize them all, but you can look up any mill symbol through the Concrete Reinforcing Steel Institute (CRSI), which maintains a registry of producer marks.
- Second mark: bar size. This is a number that tells you the bar’s diameter. In the imperial system, the number roughly equals the diameter in eighths of an inch. A #4 bar is 4/8 inch (0.500 inches) in diameter. A #8 bar is 8/8 inch, or exactly 1 inch.
- Third mark: steel type. This is a letter. “S” means carbon steel, the most common type. “W” means low-alloy steel, which is specifically designed to be weldable.
- Fourth mark: grade. This indicates the bar’s minimum yield strength and appears either as a number or as a system of lines (explained below).
What the Bar Size Number Means
The size number stamped on the bar corresponds to a specific diameter. For smaller sizes (#3 through #8), the rule is simple: divide the bar number by 8 to get the diameter in inches. A #3 bar is 3/8 inch (0.375 inches), a #5 is 5/8 inch (0.625 inches), and a #6 is 3/4 inch (0.750 inches). Larger bars break from this clean pattern. A #9 bar is 1.128 inches in diameter, a #10 is 1.270 inches, and an #11 is 1.410 inches.
If you see a two-digit number like 10, 13, 16, 19, 22, 25, 29, 32, 36, 43, or 57 on a bar, you’re looking at soft metric sizing. These are physically identical bars, just labeled in millimeters instead of eighths of an inch. A #10 metric bar is the same bar as a #3 imperial bar, both with a 9.5 mm (0.375 inch) diameter. A #25 metric bar is the same as a #8 imperial bar at 25.4 mm (1.000 inch). There is no physical difference between the two, only the number stamped on the surface.
Here are the most common size equivalents:
- #3 (metric #10): 0.375 in / 9.5 mm
- #4 (metric #13): 0.500 in / 12.7 mm
- #5 (metric #16): 0.625 in / 15.9 mm
- #6 (metric #19): 0.750 in / 19.1 mm
- #7 (metric #22): 0.875 in / 22.2 mm
- #8 (metric #25): 1.000 in / 25.4 mm
- #9 (metric #29): 1.128 in / 28.7 mm
- #10 (metric #32): 1.270 in / 32.3 mm
- #11 (metric #36): 1.410 in / 35.8 mm
- #14 (metric #43): 1.693 in / 43.0 mm
- #18 (metric #57): 2.257 in / 57.3 mm
Steel Type: “S” vs. “W”
The third mark is a single letter identifying the steel specification. The two you’ll encounter most often are “S” and “W.”
An “S” means the bar is carbon steel manufactured to the ASTM A615 specification. This is the standard rebar used in the vast majority of concrete construction. It’s strong and economical, but not specifically designed for welding.
A “W” means the bar is low-alloy steel made to the ASTM A706 specification. This steel has controlled chemical composition that makes it reliably weldable and gives it more predictable behavior under seismic loading. If a structural engineer specifies weldable rebar for a project, you need to see that “W” on the bar. Using “S” steel in its place could compromise weld quality and violate the design requirements.
How Grade Markings Work
The grade tells you the bar’s minimum yield strength, which is the amount of force the steel can handle before it permanently deforms. In the U.S., the grade number directly equals the yield strength in thousands of pounds per square inch (ksi). Grade 60 rebar has a minimum yield strength of 60,000 psi. Grade 40 yields at 40,000 psi. Grade 75 yields at 75,000 psi, Grade 80 at 80,000 psi, and Grade 100 at 100,000 psi.
Grade 60 is by far the most common in modern construction. Grades 40 and 60 have been the workhorses for decades, while Grade 80 was added more recently to both the A615 (carbon steel) and A706 (low-alloy) specifications to meet demand for higher-strength reinforcement in larger structures.
Numbers vs. Line Systems
Grade can appear on the bar in two ways. The most straightforward is a printed number: “60,” “75,” “80,” or “100” stamped right into the steel after the type letter. If you see the number, you’re done.
The alternative is a line-based system. Instead of a number, the grade is indicated by the pattern of longitudinal lines running between the main ribs of the bar. Grade 60 bars have one continuous line in addition to the standard raised ribs. Grade 75 bars have two continuous lines. Grade 80 has three. If you see no additional grade lines and no number, the bar is typically Grade 40 (though this grade is uncommon in new work). Both marking methods are valid under ASTM standards, and some mills use numbers while others use lines, so it helps to recognize both.
Putting It All Together
Imagine you pick up a bar and see, in order along the rib: a letter “H” (the mill symbol), then the number “6,” then the letter “S,” then the number “60.” That bar was made by a mill identified as “H,” it’s a #6 bar (0.750 inches in diameter), it’s carbon steel, and it’s Grade 60 with a minimum yield strength of 60,000 psi.
Now imagine a second bar showing a different mill logo, the number “19,” the letter “W,” and two continuous lines running along the bar. That’s a metric-labeled #19 bar (same as a #6 imperial), made from weldable low-alloy steel, at Grade 75.
If you encounter a bar with markings you can’t fully decipher, the mill symbol is your key to tracing it. The CRSI maintains a directory of mill marks that lets you identify the producer and, from there, request mill certifications that spell out every property of that specific heat of steel.
Color Coatings and Special Rebar
Some rebar has additional visual cues beyond the rolled-in markings. Epoxy-coated rebar is easy to spot because it’s covered in a green or sometimes blue-green coating designed to resist corrosion in bridge decks and parking structures. Galvanized rebar has a silvery zinc coating. Stainless steel rebar looks distinctly different from standard black rebar, with a brighter, smoother surface. These coatings and materials don’t change the marking system itself. The same four marks (mill, size, type, grade) still appear on the bar surface, though they can sometimes be harder to read under thick epoxy coatings. In those cases, the bar’s color and accompanying mill certifications become especially important for identification on site.